1. Field of the Invention
The present invention relates to a communication system used to transfer data packets at a relatively low data rate. The jamming margin is a measure of the tolerance of a spread spectrum system to in-band interference.
The FCC regulations part 15 which governs the US Industrial, Scientific and Medical (ISM) band (902-928 MHz) requires the use of frequency hopping or direct sequence spread spectrum (DSSS) modulations schemes.
2. The Prior Art
Spread spectrum technology has been used in the field of radio communications to eliminate the effects of interference and to prevent eavesdropping. For a fuller description of spread spectrum technology and its implementation in communications systems; reference may be made to Spread Spectrum Systems, 2nd Edition by Robert C. Dixon, published by Wiley Intersciences (ISBN 0-471-88309-3) or Coherent Spread Spectrum Systems by Jack K. Holmes also published by Wiley Intersciences (ISBN 0-47103301-4).
The technique is adapted for use in an Automatic Meter Reading (AMR) systems in which meters to be read each have a transponder containing a receiver and transmitter. The receiver is capable of recognizing a wake up signal and the transponder is then enabled to transmit data which includes data on the meter reading and the identity of the meter. The technique of the invention is used for communications between a transponder and a transmitter of a system concentrator, but is not restricted to this application. In any AMR system a very large number of transponders will be required. The design of the transponders must be reliable and economical to produce.
AMR systems typically use the unlicensed 902-928 MHz ISM band and are required to comply with FCC regulations part 15. This band is intended for use by many users and any system designed to use this band should tolerate interference from other users of the band.
One measure of the tolerance of a system to in-band interference is the jamming margin. The jamming margin may be measured by subjecting the communication system to sine wave interference. The power of the interference is decreased to the point at which acceptable performance of the communication link is obtained. Typically, for an AMR system, this would be the level at which fewer than 10% of packets are lost. The ratio of the power of the interference to that of the desired signal from the transmitter is the jamming margin, also called Jammer to Signal ratio (J/S). Each result applies for interference at the specific frequency used for the measurement.
A composite jamming margin for the system is based on all the J/S measurements obtained as the frequency of the interference is stepped through the pass band of the system. The FCC regulations part 15.247(e) allow discarding of the worst 20% of J/S measurements and using the lowest remaining J/S measurement. Typically a system is designed to maximize the minimum J/S measurement.
To reduce the cost of the system, low tolerance oscillators may be used in the transmitter and receiver. Typically there will be a relatively large frequency offset between these oscillators and the receiver must be able to tolerate this offset and still reliably demodulate the transmitted data.
The transponder receiver must be capable of locking on to the transmitted spread spectrum signal quickly. This is required so that a large number of meters can be read in a short space of time. In DSSS schemes there is normally a two step process involving:
Search step during which the transponder receiver acquires and locks onto the transmitted signal;
Tracking step during which the transponder receiver remains locked onto the signal and demodulates the received signal.
To solve the technical necessity for maximum system jamming margin, rapid acquisition, tolerance to frequency offset and economical production costs, the transmitter and receiver defined in the claims are employed.
A transmitter for extending the jamming margin of a DSSS communications system suitable for use in an AMR system, in accordance with the invention comprises: a primary PN generator for producing a primary PN chip sequence; a secondary PN generator with a clock rate less than half the repeat rate of the primary PN generator; a logic gate to combine the outputs from the primary and secondary PN generators into an output signal; a Binary Phase Shift Keying (BPSK) modulator for receiving said output signal; a data input for a digital signal representing information to be transmitted; a Manchester encoder for receiving the data signal, the data rate being substantially less than the clock rate of the secondary PN generator and producing a data output; and means for modulating the primary PN sequence with the data output.
Preferably when such a system is used in an AMR application the system transmitter used is essentially a BPSK modulated spread spectrum transmitter. The transponder receiver used is essentially a single correlator non-coherent spread spectrum receiver using a tau-dither tracking loop.
A further advantage of the use of the secondary spreading provided by the secondary PN generator is to reduce the peak spectral density which makes it easier to meet the FCC part 2 regulations.